ADDRESSING 101
1. What is in an address ?
An address is a unique “computer-understandable” identifier. Uniqueness
is defined in a domain – outside that domain, to retain uniqueness, one needs
to have either a larger address space, or do translation.
An address should ideally be valid regardless of the location of the source,
but may change if the destination moves. Fixed size addresses can be
processed faster.
Address is fundamental to networking. There is no (non-trivial) network
without addresses. Address space size also limits the scalability of networks.
In connectionless networks, the most interesting differences revolve around
addresses. After all, a connectionless net basically involves putting an
address in a packet and sending it hoping it will get to the destination.
Usually we think of layer 2 or layer 3 when we think of addressing. But,
even applications ultimately need addresses (which is why transport
protocols exist to provide port-numbers). In fact addresses may refer to a
node or an interface (latter more common).
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2. What is in a name ?
A name is a unique “human-understandable” identifier.
A name is also ideally something that is location-independent both with
respect to the source and destination. In other words, it will remain
unchanged if either the source or destination moves. (Note that in this
latter sense alone, IEEE 802 “addresses” can be considered as “names”)
In practice, names can be variable length with no limit on length. This means
that the name space is infinite, but address spaces are finite. Internet names
still have a little bit of complexity and can be pretty long. Lot of work in
middleware today is centered around specifying logical names (“the nearest
printer on this floor”) and worrying about mapping within the infrastructure.
Web names are a big deal because they also represent a brand. This was the
basis for a lot of cyber-squatting in the recent past.
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3. Why do you need an address?
- Consider a network.. If the network consists just two hosts with a pointto-point link, there is no need for addresses!
- If a network consists of more than two hosts, the source has to know
which destination to send the packet to. Immediately we need a unique
identifier for destinations.
- Similarly at the lower layer, if a host (or an intermediate node which we
call a router) is multi-homed, i.e. it has more than one interface, it faces
the problem of deciding which interface to send the packet to. For this
you need a unique index (identifier) in a packet, or infer the index from
the timing position (eg: in a T-1) line. In the latter case, someone else
(application or the higher layers have set up the timing from a knowledge
of addresses.
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4. Why do you need both a name and an address besides the ease of
understanding?
- When you have more than one address or a name and a set of addresses,
you have a level of indirection.
- Indirection gives you flexibility because names can be
organized/administered independent of addresses. For example, if a
computer moves within an organization (naming domain) but into a
different subnet, you can change addresses, but need not change the
name. Just change the name-to-address translation.
- Moreover putting names in data or signaling packets is inefficient.
- This is because names are variable length usually. Variable length
names in headers imply unbounded overhead.
- It is also complicated for router to look up a name in a routing table
because of the processing complexity. Therefore it is convenient to
use fixed length identifiers called “addresses”.
- The telephone network and ATM address may be variable length –
but the extra complexity of parsing variable length addresses is done
only during signaling and VC/circuit setup. Data/voice sample
transfer use either no identifiers (telephony) or shorter fixed length
identifiers called labels. The label space is a 32-bit space is managed
independently by switches just like frequency space is managed in a
cellular network – this leads to excellent scalability in addressing.
- But you don’t “need” names.
- For example, the telephone network has only one address, the
“phone-number.”
- You don’t dial a “name” on your telephone (except for cryptic names
which are directly mapped to numbers on your telephone itself and
not hidden from you)!
- The name-to-address mapping is provided via a parallel directory
service. The reason this was done is because of the dumbness of the
telephone – it is not intelligent to map names to addresses.
4a) What are the other issues about size of addresses ?
- A large address space allows a large network, i.e. it is fundamentally
required for network scalability
- Address can have structure to encode not only the identity of the
interface (or node as in CLNP, DECnet etc), but also the identity of the
network the interface (node) is on.
- Intermediate node can forward packets to the destination “network”
instead of the destination “node” or “interface,” and the last hop router
(on the destination network) uses lower-layer forwarding to reach the
destination interface.
- This is fundamental for scaling the forwarding and routing behavior
because the number of forwarding entries for “networks” can be far
fewer than those for “nodes” or “interfaces”.
- Large address space also makes it easier to assign addresses and
minimize configuration. L3 addresses can encode L2 addresses within
them if the addresses are long enough (see below)
- Variable length addresses are the most flexible. They save bandwidth in
small networks, and don’t require committing to the address size. But
they make the header difficult to parse (like names), and can have
unbounded overhead.
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5. What is (address or name) resolution?
- Resolution is the function of mapping one address or name to the other.
This problem arises only when there are two addresses or a name and an
address that necessitates the mapping.
- Resolution is a complex and fundamental internetworking problem
in overlay networks (such as IP over ATM) and for multicast.
- Several techniques are possible for resolution such as snooping
(assumes broadcast channel), table-based resolution, closed-form
computation-based resolution, and server-based resolution.
- The Internet uses DNS for name resolution and ARP for address
resolution. ATM networks also use DNS, but resolve to ATM addresses
instead of IP addresses. The telephone network does no resolution
because it has only one level of (variable length) addresses
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6. What is forwarding ? Do we need an address for forwarding ?
- Given a packet at a node, finding which output port it needs to go to is
called “forwarding” – it is a per-node function, whereas routing may
encompass several nodes.
- The forwarding function is performed by every node in the network
including hosts, repeaters, bridges and routers.
- Forwarding is trivial in the case of a single-port node (or in a dual port
node, where the destination is on the port other than the input port) – in
this case you don’t need even addresses.
- The earliest bridges used to forward packets from collision domain to
collision domain, without filtering. This was basic layer-2 forwarding
or also called “flooding”. It is still useful and different from the
function of repeaters. For flooding, you do not require addresses.
- For regular forwarding with forwarding tables you need addresses
because the address contains a part of the location or topology
information of the destination needed for forwarding to work.
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7. Why is there a relation between addressing and routing ? What is the
difference between flat and hierarchical addressing ?
- If you need to scale, there is a relation between addressing and routing,
else there is none.
- This is one of the differences between the difference between the
design of LAN addresses (used by bridges) and IP addresses (used by
IP routers).
- Routing scales by reducing the size of the virtual network seen by
interior nodes, i.e. reduces the size of routing tables and messages
exchanged.
- For example, consider a distant galaxy. Seen from the earth, it seems
like a single node. So a simple strategy is to send messages for planets
in the galaxy in that general direction. Within that galaxy, local routes
can be found locally.
- Similarly, in the internet, routers forward packets to the destination
“network” instead of the destination “node” or “interface,” and the
last hop router (on the destination network) uses lower-layer
forwarding to reach the destination interface.
- In order to support forwarding packets to the “destination
network,” L3 addresses have structure to encode not only the
identity of the interface (or node as in CLNP, DECnet etc), but
also the identity of the network the interface (node) is on.
- The network in effect looks like a single virtual node to the
intermediate router. This means that the “address size” and
“address space” for this virtual network is also smaller – typically
subsets of the original address space. And the routers need to
maintain forwarding entries only for these networks.
- This is the essence of “address aggregation.” The notions of
address aggregation and the procedure of “forwarding to networks
instead of nodes” means that number of forwarding entries for
“networks” is far fewer than those for “nodes” or “interfaces”.
This is how L3 routing and forwarding has the potential to scale.
- Hierarchical addressing: The simple way to encode a network-ID into a
L3 address is to make a prefix of a node address the network-ID.
Network ID
Demarcator
Host ID
Hierarchical Structure of a L3 Address
- There is a demarcator (either implicitly or explicitly specified) that
indicates where the network ID ends, and where the host ID begins.
- For address aggregation and scalable routing, two things have to
happen:
a) addresses must be aggregatable (eg: hierarchical), and
b) the allocated address prefixes must be explicitly aggregated.
- Aggregatable addresses:
- Key: addresses cannot be assigned randomly!
- For addresses to be “aggregatable,” nodes in a single network must be
assigned addresses with the same prefix.
- Moreover one or more nodes have to be designated as “border
nodes” which represent the points of entry/exit vis-à-vis the network.
Distant routers find directions to reach a border node. Multiple border
nodes are used to avoid single points of failure, and to do loadbalancing of traffic entering/exiting the network.
- If distant routers “see” multiple border routers, they can choose the
appropriate border router (that leads to the shortest path) to reach the
destination prefix. In this case, we say that the “topology
aggregation” does not match the “address aggregation,” i.e., distant
routers have some limited visibility into the topology (actually
multiple entry/exit points only) to reach destinations within the
aggregated domain.
- Explicitly aggregating addresses:
- Even if nodes within a network have been assigned the same
network prefix, the border node should be explicitly configured to
advertise reachability to the prefix, rather than advertising every
individual node address.
- This explicit configuration is done at the border of every level where
addresses are aggregated.
- Aggregation does have a cost.
- You lose information – but in shortest-distance routing case we don’t
care. For QoS routing we might care.
- You do lose optimality in routing – you tradeoff time/bandwidth
resources for scalability.
- Traffic intensity at border nodes increases – and affects QoS =>
solution is to have many border nodes and split the reachability
between them… This is a load-balancing problem to be addressed in
routing.
- Flat addresses do not encode the network ID in the L3 address.
- Flat addressing simplifies address administration (because there is no
constraint of numbering nodes with the same prefix) but does not
scale (because it requires intermediate nodes to always route to
destination nodes)
- LAN (IEEE 802) addresses are hence flat in terms of routing. They do
have a 2-level administrative hierarchy (OUI and the 24-bit number
assigned by the OUI owner) – which does not correspond to the
hierarchy network physical or logical partitioning.
- For eg: the nodes assigned addresses 10.* should be reachable through
a single border router (and topologically nearby – else routing
becomes much less optimal)
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8. Address allocation: where to place demarcators in the hierarchical
address?
- Recall that hierarchical addresses have a demarcator between the
network ID and host ID, which can be “implicitly” or “explicitly”
specified.
- In the early days of IP, implicit demarcators were used at 8-, 16- or 24bit boundaries. Such addresses were called class A, class B or class C
addresses respectively (or “classful” addresses).
- The problem with implicit demarcators is that they are inflexible, and
lead to inefficient address allocation.
- First, physical networks may be much smaller than the implicit host
ID space, and hence would underutilize host-ID space.
- Second, many class-C networks would have to be announced unaggregated even if they belonged to the same organizational
network, and occupied consecutive class-C allocations. This would
lead to an explosion of forwarding
table entries in the global Internet, limiting routing scalability.
- The solution is simple: specify explicit demarcators!
- Subnet masking and supernet (CIDR) masking were the explicit
demarcation techniques used to solve the first and second issues
respectively.
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9. Is address aggregation possible only once, or can we aggregate even more
at several levels in the Internet ?
Yes, addresses can be aggregated further. With subnet-masks and supernetmasks (CIDR), IP addresses allow multiple levels of aggregation.
- If aggregation is desired at several levels (subnetwork, network, domain,
ISP etc), these levels must be hierarchically organized (i.e. in a tree
manner), and aggregation must be done from the lowest level onward.
- Address allocation must also be done hierarchically. I.e. one cannot
go to IANA (a central body) to get an address prefix (or address
block) -- one needs to go to their ISP which provides their “transit”
into the Internet. This is known as provider-based address allocation.
- For example, top-tier ISP can get a
- The Internet today largely does provider-based address allocation. But
there are two issues which conflict with continued address
aggregation in the Internet.
- 1. The need for redundancy/availability:
- Users typically like to have connectivity to multiple ISPs for
simple redundancy and/or load balancing.
- This breaks the addressing hierarchy because the extra
links/connectivity introduces loops ! BGP policies have to deal
with this sticky issue.
- Users may advertise a block of addresses allocated by provider
1 to provider 2, so that they can get redundancy in case provider
1 goes down.
- 2. Network migration and renumbering: If a user network moves
from one network to another and does not renumber its network
nodes, aggregation cannot be done !
- This is an irritant to enterprise networks who would like the
flexibility to switch ISPs.
- NAT, DHCP, prefix-lifetimes (eg: in IPv6) help dealing with
the renumbering problem (discussed later)
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10. What is the problem with static network address boundaries? What is
subnet and supernet masking?
- Addresses have a simple 2-part structure: network number and a host
number even for arbitrary degrees of hierarchy.
- This is because any intermediate router needs to see only a two-level
hierarchy even if there are many levels, provided the topology is also
organized roughly as a hierarchy.
- The router does not see the lower (or higher) levels of the hierarchy
unless it needs to pick one among many border routers.
- So, we could inform the router on the bit position where the split
occurs (for a two-level hierarchy).
- Where and how do we demarcate the address bits ?
- Static (or implicit) demarcation wastes the address space (and limits
scalability because we have a finite address space), even if we have
multiple static classes (eg: the class A, B, C… etc). The class
structure is also too coarse.
- Solution: explicit demarcators.
- The notation 201.10.0.0/21 explicitly informs the router that the
split occurs in bit position 21. The number 21 can be encoded in 5bits (because the maximum demarcator position is 32)
- Alternatively, we could have 32-bit mask that specifies the bits
that belong to the network number, and the bits that belong to
host number. In practice, the network number bits are represented
in the mask as a contiguous set of ones starting from the leftmost
bit.
- This is the concept of a “subnet mask” or a “supernet mask” -essentially serving as explicit demarcators of the network ID
within the address.
- Subnet mask is used within an organization (autonomous
system) and supernet mask is used between autonomous
systems. As a result Internet addressing today is “classless.”
- An interesting aspect of subnet/supernet masks is that masks are NOT
carried in data-packet headers. I.e. they are exchanged only in
routing messages, and configured into individual routers.
10a. What is the difference between subnet and VLSM ?
Basic subneting: refers to a fixed demarcation strategy (mask) in addition
to natural mask (i.e. class A, B etc). I.e. only a single mask (eg::
255.255.255.0) can be used for all networks covered by the natural mask.
VLSM: (Variable length subnet mask)
Multiple different masks possible in a single class address space.
(255.255.255.0 and 255.255.254.0 could be used to subnet a single class B
address space).
- Solves/controls internal fragmentation issues (see below) and makes
address space usage more efficient
Address allocation vs Memory allocation:
In general, the problems of address space allocation are similar to that of
memory allocation to processes in operating systems. In the latter case, the
problems include:
- Internal fragmentation which refers to wastage within an allocated
block (occurs in paging systems)
- External fragmentation which refers to wastage after blocks have been
allocated (occurs in segmentation systems)
- The solution to external fragmentation is called “compaction” which
means that the allocations are taken back and reallocated in a
contiguous manner. This way, the free space is also contiguous.
- In address allocation, internal fragmentation is solved using VLSMs.
- External fragmentation is solved using CIDR (or supernetting) which
allows the allocation of arbitrary address blocks (and prefixes) to
networks and enables hierarchical address space allocation.
- Renumbering systems (DHCP) and network address translation (NAT)
help in compaction of the address space when there is a high degree of
external fragmentation.
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11. What is Classless Inter-Domain Routing (CIDR) and supernetting ?
- CIDR was developed to allow the “subnetting” (flexible address space
division between network address and host address) idea to be extended
to the network part of the IP address too.
- This effectively would make the addressing “classless” for the
purposes of routing. Since inter-domain routing protocols are the ones
that provide routing to the network-part of the IP address, the protocol
is called CIDR.
- Supernetting is essentially the idea of explicit demarcation applied to
the network ID field -- so that flexible and arbitrary sized address
chunks can be allocated to autonomous systems (which can then be
internally allocated using subnet masking)
- Implications of being “classless”: syntax and semantics
- In CIDR, we prefer the use of the term "prefix" over "network"
because it's more clear that no Class is being implied.
- Notation: 128.221.13.20/20 emphasizing that 20-bits are the network
address. This is also called the <prefix.length> notation.
- Eg: 198.213.0.0/16 has 16-bit mask shorter than natural 24-bit
(class C) mask. The 16-bit prefix is invalid in the class C (natural
mask) sense because the class C natural mask has 24-bits.
- Eg: 198.24.0.0/20 and 198.24.56.0/21 {"more specific"} can be
aggregated as 198.24.0.0/18 {"less specific"}. Note that between
each pair of dots, there are 8 bits. /20 means four bits into the third
number.
- The masks can be on arbitrary bit boundaries and don't have to be
on byte boundaries (like the earlier classful boundaries). Note that
this still means that networks have to have address spaces that are a
power of 2.
- Note: The dotted-decimal notation is confusing when we try to
interpret the CIDR masks with the prefixes ending on non-octet
boundaries.
- Longest-prefix matching: since a routing table can now contain
several addresses where part of the prefix would match, the
forwarding algorithm has to be modified to match the destination IP
address with longest prefix, and not just the first prefix. In other
words, it has to match the most-specific prefix.
- Implications on inter-domain routing and address allocation:
- Pre-CIDR, every class A, class B and class C network needed to be
advertised. The number of class B networks and class C networks is
huge (64K class B and 2^24 class C networks !!).
- The key idea is that the CIDR <prefix.length> notation allows all
the more specific routes handled by an ISP to be summarized into
one aggregate, provided that they all refer to non-overlapping
subsets of a larger address block.
- This means that a provider could get a large address block, i.e. a small
prefix (eg: 10 bit prefix rather than a 16-bit class B or 24-bit class C
prefix), and allocate smaller blocks to its customers.
- But each of these customer sub-blocks (longer prefixes) need not be
advertised. Only the single small (10-bit) prefix needs to be
advertised.
- Customers however don’t like provider-based addressing because
when they move from one ISP to another they need to renumber to
remain in the other ISP's CIDR block. This can be a complex and
costly administrative task if DHCP or NAT is not available.
- Renumbering can be done with DHCP automatically. NAT allows
avoidance of the renumbering problem by use of private address
space.
- This above allocation procedure of the provider getting large address
blocks (small prefixes) from IANA and allocating sub-blocks is called
provider-based address allocation.
- In the pre-CIDR era, sites would go to a centralized registry (IANA) to
get an address prefix which:
- A) does not take into account where that site connects to the Internet),
and
- B) allocates only classful prefixes
- The crux of CIDR is that the Internet's generally hierarchical topology
and administration is now being reflected in the addressing.
- To appreciate this fact, observe that in IEEE 802 addresses, the OUI
field also implied an administrative hierarchy, but not a topological
hierarchy (the IEEE address space is “flat”).
- The CIDR-based IP addresses are now overloaded: they have both
an administrative significance and a topological significance, and
both are expected to be hierarchical !!
- CIDR costs:
- CIDR improves scalability, at the cost of inter-domain routing
protocols also carrying subnet masks
- CIDR also requires a change in the forwarding algorithm. (need a
longest-prefix match rather than unique prefix match). The innovative
data structure used for speeding up lookup is called a “trie” which is
a tree where intermediate nodes denote part of a prefix!
- CIDR also splits the top level of addresses on a geographic boundary
which is not good for providers whose networks might span multiple
countries.
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12. Other issues in scaling of addresses ? [ADVANCED]
- Fixed sized addresses run into a scaling problem because the address
space is finite.
- Inefficient address allocation adds to this problem: eg: IP’s 32-bit
address space.
- Alternative: large, variable-sized addresses (eg: ATM, CLNP have varsized addresses with maximum length of 20-bytes)
- This can mean high per-packet overhead, and complex packet
processing.
- ATM tackles the issue of per-packet overhead by using only these
large addresses only during signaling, and resorting to small (32-bit)
fixed-size labels in the data-path.
- Alternative: large, fixed-sized addresses (eg: Ipv6 has 16 byte address,
IPX has 10-byte addresses)
- Cost: higher per-packet overhead
- Gain: additional flexibility for auto-configuration
- Alternative: multiplexing (share) the costly/scarce address space.
- DHCP is a configuration protocol that has the concept of a “lease”
(soft state). Soft state or lease means that you get an address space for
a defined period of time. You can multiplex a set of addresses as long
as the user population at any instant does not exceed the set of
addresses.
- The key issue here is that nodes do not “own” addresses, but can only
“rent” or “lease” them.
- Alternative: private-public addresses and NAT:
- Use public-addresses for global connectivity and private-addresses for
intra-AS connectivity. Private addresses need be unique only within
an AS.
- Network address translation (NAT) maps private addresses to public
addresses at the border of an AS on a packet-by-packet basis. This
assumes that a public-private address mapping is setup earlier (eg:
statically or at the beginning of the flow).
- NAT, like any translation scheme is not perfect partly because of the
interdependencies between upper layer protocols like TCP, ftp, ICMP
and IP addresses.
- Specifically, IP addresses may be present in transport and application
level data
13. Auto-configuration requirements: [ADVANCED]
Configuration revolves around how addresses (primarily) are constructed.
Typically end systems only know their hardware (IEEE 802) address…
7 things to (auto-) configure:
1. End systems need Layer 3 address, masks
2. Router finds Layer 3 addresses of end systems
3. Router finds Layer 2 addresses of end systems
4. End systems find a (default) router, name server
5. End nodes on the same LAN discover that they can send directly to each
other
6. End systems find the best router for exit traffic
7. End systems communicate on a router-less LAN
Techniques:
1. Manually configure hosts and routers {DECnet}
2. Manually configure routers only {IP, IPv6, IPX, Appletalk (seed router),
CLNP}
3. DHCP server {IP, IPv6 (optional)}
4. ARP {IP, Appletalk}
5. IEEE LAN address embedded in host-ID {IPX,CLNP,IPv6 (EUI)}
6. LAN addr = HIORD + L3 addr. HIORD is a fixed extension of the L3address to form the L2 address {DECnet}
7. Explicit “ES-Hellos” and “IS-Hellos” {CLNP, DECnet}
8. Snoop on RIP traffic for router info {Appletalk, IPX}
9. Best-exit inferred from rcvd traffic {DECnet, Appletalk}
10. Redirects for best-router only (IP, IPv6, IPX)
11. Redirects for best-router and direct end-node (CLNP)
12. Intra-LAN flag for direct end-node (DECnet)
{Needs an understanding of addressing differences in various protocols.
Covered in more detail in slide set on comparisons between IP, IPX,
CLNP, DECnet, Appletalk etc}
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Summary: Addressing requirements:
- Unique
- Address size defines max network size (eg: universal addressibility)
- Usually fixed length in packet headers, but some protocols like CLNP
have variable-length addresses.
- Aggregatable: address encodes network ID (aka “locator”)
- Allocation flexibility, efficiency and ease
- Auto-configurable (L2 address, L3 network part, host part; addresses of
router etc)